EP3901714B1 - Method for verifying the authenticity of electronic modules of a modular field device of automation technology - Google Patents

Description

The invention relates to a method for checking the authenticity of electronic modules of a modular field device in automation technology.

In process automation as well as in production automation, field devices are often used to record and/or influence physical, chemical or biological process variables. Measuring devices are used to record the process variables. These are used, for example, for pressure and temperature measurement, conductivity measurement, flow measurement, pH measurement, level measurement, etc. and record the corresponding process variables pressure, temperature, conductivity, pH value, level, flow, etc. Actuator systems are used to influence the process variables. Examples of actuators are pumps or valves that can influence the flow of a liquid in a pipe or the level in a container. In addition to the measuring devices and actuators mentioned above, field devices also include remote I/Os, radio adapters or general devices that are arranged at the field level. In connection with the invention, field devices are all devices that are used near the process or system and that provide or process process- or system-relevant information.

Corresponding field devices usually consist of a large number of electronic modules, such as plug-in modules with circuit boards, sensors with digital connections, etc. If an electronic module is replaced or added, there is currently no check to see whether the electronic module is authentic. Currently, an electronic module is usually visually checked and accepted as authentic after a positive visual inspection.

The procedure described above poses a considerable safety risk: Since there is basically no way of detecting a possibly manipulated electronic module of any kind, there is a risk that a possibly manipulated electronic module will be installed in an automation system. If the electronic module does not meet the requirements for use in an area at risk of explosion, for example, but is used in such an area, this can have life-threatening consequences.

The present patent application describes a method for ensuring module authenticity: Is the module really the module it claims to be? The primary aim here is to check whether a certain module is present, whereby the identity is checked and modules of the same construction are not automatically accepted. In a patent application filed by the applicant in parallel to this patent application, the manufacturer authenticity is checked, ie whether an electronic module comes from an original manufacturer or from a trustworthy third party or a supplier. Of course, both methods could also be used simultaneously or one after the other to check an electronic module.

The EP 2 806 599 A1 discloses a method for adding a wireless device to a wireless network using first and second certificates and first and second public keys.

The EN 10 2017 111928 A1 comprises a method for the authorized updating of a first operating software of a field device, wherein an authentication check of a second operating software for the field device, signed by means of a first private key assigned to the system, is carried out, wherein, in the event that the authentication check was carried out successfully, the first operating software located on the field device is at least partially replaced by the second operating software.

The US 2004/00327 A1 discloses a method in which a vehicle authenticates a potential component for use in the vehicle by obtaining a certification from a certification authority that an authentic component is associated with a cryptographic key. The certification attests that the cryptographic key is bound to information identifying the authentic component. The vehicle uses the cryptographic key obtained from the certification authority in cryptographic communication with the potential component and determines whether the potential component is the authentic component based on whether the cryptographic key is successfully used in the cryptographic communication.

The invention is based on the object of automatically detecting a non-authentic electronic module.

The object is achieved by a method for checking the authenticity of electronic modules of a modular field device of automation technology according to patent claim 1.

It is therefore checked whether the individual modules are present that should be present according to the Module Trust List. If an electronic module is replaced or added, this is detected using the method according to the invention. Integration into the operation is refused if the electronic module cannot prove its authenticity.

According to the invention, a field device checks whether the public key of the electronic module is included in the list of electronic modules marked as trustworthy before it includes a replaced or added electronic module in the communication required to operate the field device. The authenticity of an electronic module is usually checked during the runtime of the field device.

The key pair assigned to each electronic module is also referred to as the cryptographic identity of the electronic module. Symmetric encryption and asymmetric encryption are known in principle. While in symmetric encryption, encryption and decryption are carried out with an identical key, in asymmetric encryption this is done with two different keys.

Asymmetric cryptography often uses RSA-based key pairs that may differ in key length. Currently, RSA keys with a length of 2048 bits are already considered critical; those who need more security use key lengths of 3072 or even 4096 bits. However, the increasing key lengths not only have a negative impact on the storage space required, but also also the performance - both in asymmetric encryption and decryption and, above all, in key pair generation. Significantly more efficient than the RSA cryptosystems based on prime number fields are those that use elliptic curves. A few EC (elliptic curves) have become established. One of them is Curve25519.

Preferably, an asymmetric key pair is used in conjunction with the invention. The asymmetric encryption methods are considered to be very secure because two keys are used that cannot be derived from one another: a public key for encryption and a private key for decryption, or vice versa. The private key always remains with the creator of the key. So either the private key is used for encryption and the public key is used for decryption, or vice versa.

Furthermore, the following procedural step is proposed:
To check whether the electronic module is in possession of the public key of the appropriate key pair, the field device or the unit communicating with the field device requests the public key of the replaced or added electronic module and checks whether the public key of the electronic module is stored in the list of public keys classified as trusted.

In addition, a test is carried out to determine whether the electronic module is in possession of the private key of the appropriate key pair. According to the invention, a challenge-response procedure is used for this test. The fact that an electronic module delivers a trustworthy public key does not prove that this is also the public key belonging to this electronic module. After all, it could also be a fake module that uses an illegally obtained public key. It must therefore be checked whether this electronic module is authentic, i.e. whether the public key supplied actually belongs to this electronic module, whether the electronic module has supplied the correct public key associated with it and whether it can prove this. As mentioned, the challenge-response procedure is preferably used for this proof.

To do this, the field device or an electronic component sends any message to the replaced or added electronic module requesting the creation of a signature ("challenge"). The module signs this message and sends the signature ("response") back to the field device or the requesting electronic module. The field device or the requesting electronic module can now use the signature to check whether the electronic module is in possession of the correct private key.

The signature is created as follows: Module k applies a hash procedure to message m and encrypts the received hash value with its private key. The field device decrypts the received signature with the module's public key and compares this with a self-calculated hash value of the sent message. Ideally, both hash values are identical, which proves that a) the module told the truth because it sent the correct public key and b) that it can also prove this because it has the associated private key. By providing this proof, the replaced or added electronic module is considered authentic. Special algorithms (DSA ECDSA, etc.) have also become known for creating signatures, but these ultimately also work with an asymmetric key pair.

If an electronic module does not have a suitable key pair or only has one that is based on a different curve or on a different cryptosystem, it cannot participate in the challenge response process. This can be remedied if this electronic module has a generator that can be used to generate such a suitable key pair; alternatively, it must have a corresponding interface and a key memory so that an externally generated key pair can be subsequently written into the electronic module if necessary. In both cases, however, the module must be able to perform the appropriate/related operations, e.g. encryption with the private key.

In summary, the replaced or added electronic module - especially from the field device - is sent a random message as a challenge with the request to create a signature with the private key. The electronic module signs the message with its private key and sends the signature back as a response. The signature is used to check whether the electronic module is in possession of the private key of the appropriate key pair. Any key pair, in the sense of asymmetric cryptography, is considered suitable. RSA or EC-based key pairs are common. A key pair is a tool. Such a key pair is then used by the field device to determine the authenticity of the electronic module.

"Suitable" can be restricted in a specific case: Both the field device and the electronic module must be able to perform the respective operations (encryption, decryption) with the key pair. If the field device can only do EC and the module can only do RSA, for example, then the invention will not work. If the electronic module cannot do asymmetric cryptography at all, then no suitable key pair exists.

Some special cases are described below: If the check shows that the replaced or added electronic module does not have a key pair, a check is carried out to see whether a key pair can be generated or provided for the electronic module,
In the event that the key pair is provided or generated by another electronic module, the key pair is passed on to the replaced or added electronic module.

Furthermore, in connection with the invention, it is proposed that a replaced or added electronic module which does not have a suitable key pair or for which no suitable key pair can be generated remains excluded from communication.

According to the invention, if the verification shows that the replaced or added electronic module has a key pair, but that the public key of the key pair is not stored in the list, although the electronic module appears to be authentic, the public key of the generated key pair is assigned to the list of electronic modules classified as trustworthy as soon as an authorized person has confirmed the trustworthiness of the electronic module.

Even if a suitable key pair can be generated for the electronic module, the public key of the key pair is stored in the list of electronic modules classified as trustworthy if an authorized Person confirms the trustworthiness of the electronic module. This allows the list to grow and contain the public keys of several electronic modules. Of course, when replacing modules, it is a good idea to remove the public key of the replaced module from the Module Trust List.

If an electronic module does not have a suitable key pair or only has one that is based on a different curve or on a different cryptosystem, it cannot participate in the challenge-response process. In order to generate a suitable key pair, this electronic module must have a generator with which such a (suitable) key pair can be generated, or it must have an interface and a key memory so that an externally generated key pair can be written into the electronic module. In both cases, however, the electronic module must master the appropriate / associated requirements and operations (e.g. encryption with the private key).

It is intended that the electronic modules are each provided with a suitable key pair by the original manufacturer or a third party authorized by the original manufacturer during the production process or during a service call; furthermore, the public key of the suitable key pair is stored at the corresponding time in the list of electronic modules classified as trustworthy. During production or later when modules are exchanged or added, the field device is informed by a trusted person that the exchanged or added electronic module is to be considered trustworthy. The field device then transfers the public key of the electronic module to its Module Trust List MTL.

When an electronic module is replaced, the public key of the replaced electronic module is deleted from the list of electronic modules classified as trusted.

As mentioned previously, the verification or test of whether the electronic module is authentic can be carried out while the field device is in operation.

It has also already been mentioned that in connection with the invention, a derivation, e.g. a hash value, or another independent and unique identification can be used instead of the public key of the electronic module.

• Fig. 1: eine schematische Darstellung eines Feldgeräts mit mehreren elektronischen Modulen, das zur Durchführung des erfindungsgemäßen Verfahrens geeignet ist, und
• Fig. 2: ein Flussdiagramm, das das erfindungsgemäße Verfahren mit unterschiedlichen Weiterbildungen beschreibt.

The invention is explained in more detail with reference to the following figures. It shows:

• Fig.1 : a schematic representation of a field device with several electronic modules, which is suitable for carrying out the method according to the invention, and
• Fig.2 : a flow chart describing the method according to the invention with different further developments.

Fig.1 shows a schematic representation of a field device FG with several electronic modules Mk, which is suitable for carrying out the method according to the invention. In the case shown, the field device FG has three electronic modules Mk with k = 1, 2, 3. Each electronic module Mk of the field device FG is assigned a suitable key pair Pk, pk with k = 1, 2, 3. This suitable key pair Pk, pk is a prerequisite for the associated electronic module Mk to be able to confirm its authenticity. Each key pair Pk, pk consists of a public key Pk and a private key pk. Furthermore, the public keys Pk of the suitable key pairs Pk, pk are stored in a list MTL, whereby the MTL list is assigned to the field device FG or to a unit U communicating with the field device FG. MTL is the abbreviation for Module Trust List. The list contains the public keys Pk of the electronic modules Mk classified as trustworthy. Only if the test steps according to the method according to the invention and/or its further embodiments are positively answered, a replaced or newly added electronic module Mk is functionally integrated into the field device FG.

The field device is also assigned its own key pair Q, q consisting of public key Q and private key q. The field device FG can, if required, transmit the public key Q to one or more electronic modules Mk in order, for example, to determine secret knowledge between the field device FG and the electronic module Mk and to use this (or a derivation of it) as a symmetric key for encrypted communication (keyword: "Diffie-Hellman", exchange of public keys). It is also possible that not only the electronic module Mk has to identify itself to the field device FG, but also the field device FG to the electronic module Mk. If an electronic module Mk has stored a lot of sensitive (secret) data, for example, it may only be able to communicate this to one or only certain field devices FG. For this, each electronic module Mk would have to have a stored field device trust list in which the public keys of the field devices FGk classified as trustworthy are listed.

Fig.2 shows a flow chart describing the method according to the invention with different developments.

Under program point 10, a new electronic module Mk, e.g. Mod3new, is plugged in instead of, for example, the electronic module Mod3; alternatively, a new module Mk, e.g. the electronic module Mod4, is added. Under program point 20, a check is made as to whether the new electronic module Mk has a suitable key pair Pk, pk. If this is the case, a check is made under program point 30 as to whether the public key Pk of the replaced or added electronic module Mk. Mk is listed in the MTL list of public keys Pk. If the check is positive, a check is made at program point 40 as to whether the new electronic module Mk has the correct private key pk. If this check is positive, communication or interaction between the replaced or added electronic module Mk and the field device FG or another electronic module Mk of the field device FG relating to the functionality of the field device FG is permitted. The check ends at program point 60. It is also possible that the check is carried out by a separate unit. This is in the Fig.2 not shown separately.

In order to check whether the electronic module Mk is in possession of the public key Pk of the appropriate key pair Pk, pk - which is determined at program point 30 - the field device FG or the unit U communicating with the field device FG requests the public key Pk of the replaced or added electronic module Mk and checks whether the public key Pk of the electronic module Mk is stored in the MTL list.

The check to see whether the electronic module Mk is in possession of the correct private key pk of the appropriate key pair Pk, pk (program point 40) is carried out using a challenge-response procedure. For this purpose, any message m is sent to the replaced or added electronic module Mk, in particular from the field device FG, as a challenge, with the request to create a signature with the existing private key pk. The electronic module Mk signs the message m with its private key pk and sends the signature back as a response. The signature is used to check whether the electronic module Mk is in possession of the correct private key pk of the appropriate key pair Pk, pk. This is the case if the message m is the message m again after encryption and decryption.

Let's look at what happens if the checks under one of the program points 20, 30 or 40 produce a negative result.

If the check under program point 20 shows that the electronic module Mk does not have a suitable key pair Pk, pk, a check is carried out to see whether a key pair Pk, pk can be generated or provided for the electronic module Mk (Program point 70). If the key pair Pk, pk can be provided or generated by the field device FG or another electronic module Mk (program point 80), the key pair Pk, pk is passed on to the replaced or added electronic module Mk. It is also possible that the replaced or added module Mk generates a suitable key pair Pk, pk itself. To do this, it must have the appropriate technical requirements. The public key Pk is stored in the MTL list as soon as an authorized person has confirmed the trustworthiness of the electronic module Mk.

If the electronic module Mk does not have a suitable key pair Pk, pk or if no suitable key pair Pk, pk can be generated for the electronic module Mk (program point 70), the electronic module Mk remains excluded from communication. If necessary, an error message is generated that the electronic module Mk does not have a suitable key pair Pk, pk (program point 90).

If the public key Pk of the replaced or added module Mk is not included in the MTL list (program point 30) and an authorized user does not confirm the trustworthiness of the electronic module Mk, an error message is issued that the electronic module Mk is not trustworthy (program point 120). The field device FG does not integrate the replaced or added module into the communication.

If the challenge response test under program point 40 shows that the electronic module does not have the correct private key pk, an error message is generated under program point 130 stating that the electronic module Mk is not authentic.

The method according to the invention makes it possible to reliably prove the correct identity of an electronic module Mk. Fake modules can be eliminated.

Claims (10)

1. Method for checking the authenticity of electronic modules (Mk) of a modular field device (FG) in automation technology

   where each electronic module (Mk with k = 1, 2, ... n) of the field device (FG) has a suitable key pair (Pk, pk with k = 1, 2....n) is assigned, which confirms the identity of the electronic module (Mk), wherein each key pair (Pk, pk) consists of a public key (Pk) and a private key (pk), and wherein the public keys (Pk) of the suitable key pairs (Pk, pk) are stored in a list (MTL), wherein

   the list (MTL) to the field device (FG) or one with the field device (FG) communicating unit (U), the method comprising the following method steps:

   - When replacing or adding an electronic module (Mk), the field device (FG) or the unit (U) communicating with the field device (FG)
   checked:

   - whether the replaced or added electronic module (Mk) has a key pair (Pk, pk), and

   - whether the public key (Pk) of the replaced or added electronic module is included in the list (MTL) of public keys (Pk),

   - whether the electronic module (Mk) is in possession of the correct private key(pk), whereby a challenge-response procedure is used to test whether the electronic module (Mk) is in possession of the private key (pk) of the suitable key pair (Pk, pk),

   - a communication or interaction of the field device concerning the functionality of the replaced or added electronic module (Mk) with the field device (FG) or another electronic module (Mk) is permitted if the check is completed with a positive result,

   - If the check shows that the replaced or added electronic module (Mk) does not have a key pair (Pk, pk), the system checks whether a key pair (Pk, pk) can be generated or provided for the electronic module (Mk), wherein, in the event that the key pair (Pk, pk) is provided or generated by a further electronic module (Mk), the key pair (Pk, pk) is transferred to the replaced or added electronic module (Mk).
2. Method according to claim 1 with the following process step:

   In order to check whether the electronic module (Mk) is in possession of the public key (Pk) of the suitable key pair (Pk, pk), the field device (FG) or the unit (U) communicating with the field device (FG) requests the public key (Pk) of the replaced or added electronic module (Mk) and checks whether the public key (Pk) is in the possession of the suitable key pair (Pk, pk).

   of the electronic module (Mk) is stored in the list (MTL).
3. Method according to claim 1 with the following method steps:

   In particular, the field device (FG) sends any message (m) as a challenge to the replaced or added electronic module (Mk) with the request for
   Signature creation with the private key (pk),

   the electronic module (Mk) signs the message (m) with its private key (pk) and sends the signature back as a response,

   The signature is used to check whether the electronic module (Mk) is in possession of the private key (pk) of the appropriate key pair (Pk, pk).
4. Method according to claim 1 with the following method step:
   In the event that the electronic module (Mk) does not have a suitable key pair (Pk, pk) or that no suitable key pair (Pk, pk) can be generated for the electronic module (Mk), the electronic module (Mk) remains excluded from communication. excluded.
5. Method according to one or more of the preceding claims, comprising the following method steps:
   If the check shows that the replaced or added electronic module (Mk) has a key pair (Pk, pk), but that the public key (Pk) of the key pair (Pk, pk) is not stored in the list (MTL), the public key (Pk) of the generated key pair (Pk, pk) is assigned to the list (MTL) if an authorized person (A) confirms the trustworthiness of the electronic module (Mk).
6. Method according to claim 1 with the following process steps:
   in the event that a suitable key pair (Pk, pk) can be generated for the electronic module (Mk), the public key (Pk) of the key pair (Pk, pk) is stored in the list (MTL) if an authorized person (A) confirms the trustworthiness of the electronic module (Mk).
7. Method according to one or more of the preceding claims, comprising the following method steps:

   - the electronic modules (Mk) are manufactured by the original manufacturer or a third party authorized by the original manufacturer during the production process or in the case of Each service insert is provided with a suitable key pair (Pk, pk), and

   and the public keys (Pk) of the suitable key pairs (Pk, pk) are stored in the list (MTL).
8. Method according to one or more of the preceding claims, comprising the following method step:
   When an electronic module (Mk) is replaced, the public key (Pk) of the replaced electronic module (Mk) is deleted from the list (MTL).
9. Method according to one or more of the preceding claims, comprising the following method step:
   The inspection and test are carried out while the field device (FG) is in operation.
10. Method according to one or more of the preceding claims, comprising the following method step:
    Instead of the public key (Pk) of the electronic module (Mk), a derivation, e.g. a hash value, or another independent and unique identification is used.

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